Several techniques are used in cellular systems to improve robustness to co-channel interference. In fractionally loaded systems, interference avoidance and interference averaging are two of the known techniques for improving system performance. With ideal interference avoidance, transmissions in the cell-of-interest are scheduled for time periods when other co-channel cells are not transmitting so that interference is avoided. In theory, interference avoidance can provide a higher total system capacity than interference averaging. However, interference avoidance requires system-level coordination of transmission schedules among the different cells.
In contrast to interference avoidance, interference averaging does not normally require any coordination among different cells. The goal of interference averaging is to make the performance impact of time-varying or bursty interference similar to that of a lower-power continuous interferer. A known method for interference averaging is burst mode transmission combined with coding and time interleaving. In this method, the data to be transmitted is first encoded using a forward error correction (FEC) code; it is then interleaved and transmitted over multiple time slots, where a large separation between the multiple slots is preferred. In addition, the transmitter is configured to turn off during any time periods where no data is scheduled for transmission. A known improvement to this method is to add frequency hopping, where the radio frequency (RF) carrier of the transmission is changed on a burst-by-burst or frame-by-frame basis.
For packet-oriented orthogonal frequency division multiplexing (OFDM) systems, the above-described method of interference averaging has room for improvement. This is primarily because the time duration of an OFDM symbol is much greater than that of a single carrier system (for the same occupied bandwidth). Because of the long symbol duration, time interleaving over several slots can produce an unacceptable delay, especially for packet data systems.
Another known method for interference averaging is direct sequence code division multiple access (DS-CDMA). With this method, transmissions are continuous in time and the spreading codes of the desired and interfering signals are uncorrelated. As a result, the interference at the output of the DS-CDMA despreader has the appearance of Gaussian noise of virtually constant average power. However, conventional DS-CDMA is not directly applicable to OFDM systems since the spreading process creates interference between the subcarriers in a multipath OFDM channel.
Thus, there is a need for improved an OFDM interference averaging technique that can achieve interference averaging without creating long delay periods and additional interference between subcarriers in a multipath channel.